This project studies the effects of morphine on the capabilities of behaving monkeys to detect temperature variations in the noxious range applied to the face. Rhesus monkeys are trained to detect small temperature increments (0.4 C, 0.6 C, 1.0 C) from a 46 C baseline. In another task, the monkeys are trained to discriminate 47C from innocuous temperature. Additionally, the animals have to perform either a visual task where the cue is the onset of a light or an innocuous thermal cooling task where they have to detect a decrease of 0.5 C from a 38 C baseline. Morphine sulfate is directly administered in the medullary dorsal horn by microinjection technique (0.2 Mul in 2 minutes). Morphine reduces in a dose-dependent manner (1 Mug - 10 Mug) the capabilities of the monkeys to detect the small change of 0.4 C from 46 C. This effect is reversed by the systemic administration of the specific opiate antagonist naloxone. Larger amounts of morphine sulfate (10 Mug) are required to attenuate the detection of 0.6 C and 1.0 C. In addition the same dose of 10 Mug is able to suppress the discrimination of 47 C. Even with large amounts of (10 Mug and 15 Mug) morphine does not affect the capabilities to detect the onset of a light or a small 0.5 C decrement in the cooling range. These data suggest that morphine can specifically attenuate noxious thermal discrimination when directly applied to the medullary dorsal horn without altering detection and perception of innocuous thermal or visual stimuli.